Build the NAddIn project to create bin\debug\NAddIn.dll. Test the Add-In in Excel: Open a new workbook in Excel. Select Tools, Add-Ins, Automation. NAddIn.Functions should be listed - select it. OK. In a cell, type =Add2(3,4) The cell should display 7. To register the .dll after moving it, run regasm with the /codebase flag (typically as c:\WINDOWS\Microsoft.NET\Framework\v1.1.4322\RegAsm /codebase NAddIn.dll). You will get a warning about the assembly being unsigned - you can ignore this (or sign the assembly as documented).